What Really Happened in the Late Triassic?
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Historical Biology, 1991, Vol. 5, pp. 263-278 © 1991 Harwood Academic Publishers, GmbH Reprints available directly from the publisher Printed in the United Kingdom Photocopying permitted by license only WHAT REALLY HAPPENED IN THE LATE TRIASSIC? MICHAEL J. BENTON Department of Geology, University of Bristol, Bristol, BS8 1RJ, U.K. (Received January 7, 1991) Major extinctions occurred both in the sea and on land during the Late Triassic in two major phases, in the middle to late Carnian and, 12-17 Myr later, at the Triassic-Jurassic boundary. Many recent reports have discounted the role of the earlier event, suggesting that it is (1) an artefact of a subsequent gap in the record, (2) a complex turnover phenomenon, or (3) local to Europe. These three views are disputed, with evidence from both the marine and terrestrial realms. New data on terrestrial tetrapods suggests that the late Carnian event was more important than the end-Triassic event. For tetrapods, the end-Triassic extinction was a whimper that was followed by the radiation of five families of dinosaurs and mammal- like reptiles, while the late Carnian event saw the disappearance of nine diverse families, and subsequent radiation of 13 families of turtles, crocodilomorphs, pterosaurs, dinosaurs, lepidosaurs and mammals. Also, for many groups of marine animals, the Carnian event marked a more significant turning point in diversification than did the end-Triassic event. KEY WORDS: Triassic, mass extinction, tetrapod, dinosaur, macroevolution, fauna. INTRODUCTION Most studies of mass extinction identify a major event in the Late Triassic, usually placed at the Triassic-Jurassic boundary. For example, Raup, and Sepkoski (1982) noted the end-Triassic event as one of the five major mass extinctions during the Phanerozoic, equivalent in scale to the Late Ordovician, Late Devonian, and end- Cretaceous events. For marine invertebrates, Sepkoski (1982) estimates a loss of 20% of families, which scales to more than 50% loss of species. The purpose of this paper is to review previous opinions on what happened, to focus on the question of timing of the event, or events, in the Late Triassic, and to consider briefly the implications for theories of periodicity of extinctions. FORMER VIEWS Downloaded by [The University Of Melbourne Libraries] at 04:14 11 March 2013 Extinctions The Late Triassic event was recognised among terrestrial vertebrates as early as the 1940s. Colbert (1949) described the extinction of a whole range of groups on land, such as the labyrinthodont (i.e. temnospondyl) amphibians, procolophonids, prolacertiforms, rhynchosaurs, thecodontians, and most mammal-like reptiles. In the sea, dominant marine reptiles, such as the nothosaurs and placodonts, also died out during the Late Triassic. The significance of the event for terrestrial vertebrates was emphasised by the replacing animals, a range of new groups, such as the modern amphibians, turtles, sphenodontids (relatives of the lizards), crocodilians, dinosaurs, pterosaurs, mammals, and marine plesiosaurs and 263 264 M.J. BENTON crocodilians. Later publications on the terrestrial vertebrate turnover are reviewed by Benton (1986b). The Late Triassic event was also recognised among marine invertebrates. Newell (1962, 1963, 1967) and others noted the extinction of ceratite ammonoids, various bivalve groups, major groups of brachiopods, and the last conodonts. At generic and species levels, Hallam (1981) has shown that there were high levels of extinction among ammonoids and bivalves (42% loss of genera, and 92% loss of species of the latter), and Hallam (1991) provides a broader review of terminal Triassic extinctions. Timing The timing of the Late Triassic event(s) has always been in dispute. Initially, the extinction was generalised merely as "late Triassic" or "end-Triassic". The marine events, as Hallam (1981) noted, were "either at or very shortly before the end of the period", while the terrestrial vertebrate turnover took place much earlier, in Carnian times, a time difference of 12-17 Myr, depending upon which of several current time scales one accepts (e.g. Cowie and Bassett, 1989; Forster and Warrington, 1985; Harland et ah, 1989). The earlier peak of vertebrate extinction was noted by Bakker (1977), Charig (1979), Benton (1983, 1986a, b, 1987b), Olsen and Sues (1986), and Lucas (1990). In addition, Olsen and Galton (1977, 1984) argued that many of the supposed extinctions of terrestrial vertebrates in the second event, at the end of the period, were imaginary, because a number of supposedly terminal-Triassic horizons were redated as Early and Middle Jurassic. Benton (1986a) extended the vertebrate evidence, and proposed that there were two Late Triassic events, of which the first was marginally the larger overall. The two events applied to all animals—marine and terrestrial, invertebrate and vertebrate—and not just to terrestrial vertebrates, as has been supposed by some authors (e.g. Hoffman 1989a, p. 204; 1989b, p. 29). Raup and Sepkoski (1984; Sepkoski and Raup, 1986; Sepkoski, 1986, 1989) find these two Late Triassic peaks of extinction in their familial and generic data sets of marine animals, but they suggest that the Carnian peak is not a genuine mass extinction for four reasons: (1) The Carnian extinction peak is an artefact of a succeeding poor fossil record (Olsen and Sues, 1986, p. 343; Sepkoski and Raup, 1986, p. 11). It is sugggested that marine diversity peaked in the early and middle Carnian because of the presence of one or more extremely fossiliferous horizons—particularly the Cassian Beds of Italy—followed by relatively unfossiliferous horizons in the late Downloaded by [The University Of Melbourne Libraries] at 04:14 11 March 2013 Carnian and early Norian. In other words, the Carnian decline in diversity is not the result of a mass extinction, but merely a gap in the record. (2) There may have been a backward smearing of the effects of the end-Triassic extinction event (Sepkoski, 1986, p. 286). This is linked to the first argument, but with the allied suggestion that the whole record between the Carnian and the end of the Triassic is so poor that extinctions are pulled back to the start of the gap—the Signor-Lipps effect. (3) The Carnian peak in extinction was produced mainly by extinctions of ceratite ammonoids and gastropods which were undergoing high turnover at the time (Sepkoski, 1986, p. 286; 1989). The high rates of extinction are simply associated with high origination rates. LATE TRIASSIC EVENTS 265 (4) Even if there were a Carnian extinction event, it is probably a local Alpine phenomenon, and not global (Hallam, 1991). For example, Newton et al. (1987) found no evidence for bivalve extinctions at that time in the western United States, while Olsen et al. (1987, 1988) regard the end-Triassic event as the dominant one, at least in the basins of the Newark Supergroup of the eastern United States. Arguments (1) and (4) are addressed in this paper. Argument (2) is an extreme version of (1), and it is regarded as unlikely to explain much of the Carnian extinction peak since there is a time span of 12-17 Myr involved, and the Norian fossil record is probably not bad enough to allow such a major backwards smearing of the data. The high turnover argument (3) is doubtless true, although it may have something to do with (1) since it is an example of the Lagerstatten Effect, and it is not addressed separately here. EVIDENCE FOR A CARNIAN EXTINCTION EVENT Marine Animals Simms and Ruffell (1989, 1990) reviewed some of the evidence for extinctions and turnover in the sea during the mid-Carnian, and found significant extinctions in a range of higher taxa. Ceratite ammonoid diversity peaked at about 150 genera in the early Carnian, and fell to 100 or so in the Norian, with peaks in extinction rate in the early to middle Carnian. Although the major extinctions of bivalves occurred at the end of the Triassic (Hallam, 1981), Johnson and Simms (1989) noted high extinction levels among the scallops in the Carnian. Bryozoans also showed major declines in the Carnian. From a peak in the early Carnian, the group declined from 22 to 13 species in the late Carnian, and only two survived into the Norian (Schafer and Fois, 1987). According to Johnson and Simms (1989) and Simms (1991), the major post- Palaeozoic turnover among crinoids took place in the Carnian, not at the end of the Norian. Encrinids and isocrinids, typical of earlier faunas, show a dramatic reduction in diversity between the early and middle Carnian, and in the case of the encrinids, became extinct altogether. Another group, the Somphocrinidae experienced a major decline in abundance and diversity at the end of the late Carnian. The Carnian also witnessed a major decline of echinoids (Smith, 1990; Simms, 1991), and indeed they were unaffected by the end-Triassic event. Conodonts were also in decline. Although they finally disappeared at the end of the Triassic, the major drops in species and genus origination rates began in the Downloaded by [The University Of Melbourne Libraries] at 04:14 11 March 2013 early Carnian (Aldridge, 1988). Only one new genus of conodont arose in the Norian, and the final extinction at the end of the period was a whimper. Stanley (1988) has documented major turnovers and extinctions among reef organisms. Reefs were absent in the Early Triassic, whereas those of the Middle Triassic to early Carnian resembled Permian reefs, being dominated by Tubiphytes, calcisponges, bryozoans, and calcareous algae. Norian reefs, on the other hand, were very different, with scleractinian corals becoming the key framework builders. The changeover in reef type is dated as middle or late Carnian, and it involves marked diversity changes in foraminifera, sponges, corals, bryozoans, algae, and reef-living bivalves and gastropods (Stanley, 1988, p.